The present invention relates to prostheses for replacement of a portion of the knee joint. More particularly, the present invention relates to a modular prosthesis for replacement of the distal portion of the femur and the proximal portion of the tibia.
Present prostheses for the replacement of the knee joint generally include two structures. The conventional prosthesis generally includes a femoral portion that is designed to replace the distal portion of the natural femur. The femoral portion may be formed with or without an elongated stem that is designed to extend into the shaft of the femur. The conventional knee prosthesis also generally includes a tibial portion that is designed to replace the proximal portion of the tibia. Like the femoral portion, the tibial portion may be formed with or without an elongated stem that is designed to extend into the shaft of the tibia. The tibial portion normally has a bearing surface or is provided with a separate bearing element to mate with bearing surfaces on the femoral portion to permit movement between the femur and the tibia. For a discussion of the structure and use of conventional knee prostheses, reference is made to U.S. Pat. Nos. 4,470,158, 4,340,978, 4,309,778, 4,301,553, and 4,219,893.
Because conventional knee prostheses are generally provided as two devices, the practice has been to maintain a large inventory of prostheses of different sizes and configurations, both with and without elongated stems, to accommodate the different bone sizes that are to be replaced. Generally, the patient is evaluated using conventional x-ray procedures, or some other means, to determine the approximate bone size, and consequently the approximate required prosthesis size. During the replacement operation, several different prostheses are made available as suggested by the x-ray evaluation. The appropriate prosthesis is then selected for insertion into the patient at the time of the replacement operation.
One problem with the conventional knee prosthesis is that the hospital has to maintain a large number of differently sized and oriented prostheses to accommodate different sizes of bones to be replaced. Generally, a supply of prostheses with and without elongated stems must be maintained in the inventory. Also, the prostheses with different shapes and angles of the stem must be maintained. Even with this large inventory of prostheses, it is often difficult to have a prosthesis available that is sized and shaped exactly for the individual patient.
Prostheses having femoral portions and tibial portions with removable stems are known. One such prosthesis is shown in U.S. Pat. No. 4,462,120. U.S. Pat. No. 4,462,120 discloses a knee prosthesis having upper and lower members that are provided with externally threaded, tapered shanks that are detachably secured to each member. The threaded shanks are disclosed to be uniformly sized, and are shown in only one orientation, with the axis of the shank generally perpendicular to a transverse plane passing through the planar surfaces of the upper and lower members. Neither the upper member nor the lower member is configured to be implanted without the threaded shank. Also, only one size and shape of threaded shank is disclosed for use with either the upper member or the lower member. This prosthesis is thus limited in flexibility because of the necessity for inclusion of the shanks, and because the shanks are limited in structure and design.
Prostheses having limited interchangeable components of different sizes are disclosed in U.S. Pat. Nos. 4,257,129 and 4,404,691. U.S. Pat. No. 4,257,129 discloses a prosthetic tibial implant having a replaceable articulation member removably attached to an anchor member. The patent discloses that different sizes of articulation members are available and interchangeable with each other. The articulation member is secured to the anchor member by the use of a vertical pin that is secured by a clip. The patent does not disclose that the anchor member can be utilized without a stem. In fact, the stem must be a part of the anchoring member in order to receive the vertical pin to lock the articulation member in place. This prosthesis is limited in flexibility because of the requirement to utilize the anchor member with the stem.
U.S. Pat. No. 4,404,691 discloses a knee prosthesis in which different sizes of femoral components are interchangeable with different sizes of tibial components. The femoral components and the tibial components are each unitary structures, with only their sizes being different. Neither the femoral components, nor the tibial components have stems that are removable. Although the patent does disclose some degree of modularity, the flexibility is limited to different sizes of components being interchangeable with each other. The patent does not disclose that any of the femoral components or tibial components can be used without a stem attached.
One object of the present invention is to provide a modular knee prosthesis that offers a great deal of flexibility in its assembly, both as to the size and shape of the assembled device.
Another object of the present invention is to provide a modular knee prosthesis that may be assembled in the operating room before any component of the prosthesis is inserted into the patient.
Yet another object of the present invention is to provide a modular knee prosthesis in which the femoral component and the tibial component are designed to function with or without an optional stem.
Yet another object of the present invention is to provide a modular knee prosthesis that may include a stem portion that is shaped to fit a curvature within the bone structure, if necessary.
According to the present invention, a modular knee prosthesis assembly for replacement of a portion of the knee joint is provided. The assembly comprises a femoral component that is formed to include first bearing surfaces and first means for demountably attaching an optional femoral stem member. The assembly also comprises a tibial component that is formed to include a platform and second means for demountably attaching an optional tibial stem member. The assembly also includes a separate tibial insert that is configured to be supported by the tibial component platform. The tibial insert is formed to include second bearing surfaces that are configured to mate with the first surfaces on the femoral component to permit pivotal movement between the femoral component and the tibial component.
One feature of the present invention is that the femoral component and the tibial component are designed to function either with or with-out an optional stem member. One advantage of this feature is that the same tibial or femoral component can be utilized in more than one application.
In preferred embodiments of the present invention, the femoral component is selected from a group consisting of differently sized and shaped femoral components, with the group of femoral components having the first attaching means generally uniformly sized. One feature of the foregoing structure is that a number of femoral components are available, with the selection of the femoral component dictated by the needs of the patient. Each femoral component of the group has a first attaching means that is generally uniformly sized with the other components of the group. One advantage of this feature is that each femoral component in the group is interchangeably engageable with an optional femoral stem member to increase the flexibility of the assembly.
Also in preferred embodiments of the present invention, the tibial component is selected from a group consisting of differently sized tibial components, with the group of tibial components having the second attaching means generally uniformly sized. One feature of the foregoing structure is that differently sized and shaped tibial components are available, with the selection depending upon the requirements of the patient. Each tibial component in the group has second attaching means that are generally uniformly sized. One advantage of this feature is that each tibial component in the group is interchangeably engageable with an optional tibial stem member to increase the flexibility of the assembly.
Also in preferred embodiments of the present invention, the assembly includes at least one femoral stem extension that has a lower portion that is configured to be attached to the femoral stem member to increase the length of the stem attached to the femoral component. The femoral stem extension is selected from a group consisting of differently sized and shaped femoral stem extensions, with the group of femoral stem extensions having the lower portions generally uniformly sized. One feature of the foregoing structure is that the differently sized and shaped femoral stem extensions have lower portions for mating with the femoral stem members that are uniformly sized. One advantage of this feature is that any of the group of femoral stem extensions is optionally mateable with any of the femoral stem members to increase the flexibility of the assembly.
The modular knee prosthesis of the present invention thus provides the ability to assemble a custom knee prosthesis by selecting different sizes and shapes of individual components to meet the requirements of the individual patient more precisely. The interchangeability of the components of the prosthesis greatly reduces the inventory required to be maintained by the hospital Also, the interchangeability of the components greatly increases the flexibility of the system, and provides for the assembly of a knee prosthesis that may otherwise be unavailable.
Additional objects, features, and advantages of the invention will be apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.